Loz, have the three belts been changed? If so then it's a fair price if in good nick with history. If the belts have not been done, then you have paid too much as they are going for £600 upwards. In my opinion it is still a daft price for what it is. These cars are worth more broken up these days.
Let the tuning commence!
I personally would not bother with supercharging, only a couple have succeeded and that has cost them alot of time and money developing it. I nearly bought one myself about a year ago, glad I didn't now as the engine was stressed.
210-215 is probably the limit for a tuned nasp version with 270 cams. You can up this if you go the DTA route using 285 cams but gets very expensive. A quality charged version can reach 250 but at a cost. Turbo charging, forget it, you now know how much room there is in there!
The way to make the most of these cars is in the handling and weight reduction, the standard chassis is excellent already but with a few other mods, even better.